1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device including a photosensor with improved sensing efficiency, and a method of fabricating the same.
2. Discussion of the Related Art
According as various mobile electronic devices, for example, mobile phone, PDA and notebook computer are developed recently, there are increasing demand for flat panel display devices with thin profile and light weight. Examples of flat panel display devices include liquid crystal display devices (LCD), field emission displays (FED), and plasma display panels (PDP). Among the flat panel display devices, the liquid crystal display device has the great attentions owing to the technology suitable for the mass production, the easy driving method and the realization of high resolution.
The liquid crystal display device corresponds to a transmitting-type display device, which controls the light transmittance through a liquid crystal layer by refractive anisotropy, thereby displaying desired images on a screen. For displaying the desired images in the liquid crystal display device, there is a requirement for a backlight unit whose light passes through the liquid crystal layer. Thus, the liquid crystal display device is comprised of the liquid crystal panel and the backlight unit provided at the rear of the liquid crystal panel.
The backlight unit emits the light of constant luminosity to the liquid crystal panel. That is, since the backlight unit emits the light of constant luminosity even in case of the relatively bright environments, it causes the increase of power consumption. Virtually, the backlight unit uses a large percentage of total power, in more detail, about 80% or more of the total power used for driving the liquid crystal display device. In order to fabricate the liquid crystal display device of the low power consumption type, there are various methods to lower the power consumption of backlight unit.
One of the various methods to lower the power consumption of backlight unit is to provide a liquid crystal display device including a photosensor which can sense the luminosity of external light from the surroundings.
As shown in FIG. 1, a liquid crystal display device 100 including a photosensor to sense luminosity of external light from the surroundings includes a liquid crystal panel 150 provided with an upper substrate 110, a lower substrate 120, and a liquid crystal layer 130 between the upper and lower substrates 110 and 120; and a backlight 200 provided at the lower substrate 120 and emitting light to the liquid crystal panel 150. The liquid crystal panel 150 is defined with a display region to display picture images; a non-display region on which picture images are not displayed; and a black matrix region, provided between the display region and the non-display region, for blocking the light.
The upper substrate 110 corresponds to a color filter substrate. At this time, R, G and B color filters 101 are formed in the pixel region of the upper substrate 110, and black matrix films 105 are formed in the black matrix region of the upper substrate 110. Although not shown in detail, the black matrix film 105 is provided in the boundary (not shown) of pixels, thereby preventing the light leakage. The color filter 101 is a resin film including dye or color. In addition, an overcoat layer (not shown) may be formed to planarize the surface of color filters 101. On the overcoat layer, there is a common electrode 103 to apply a voltage to the liquid crystal layer 130.
The lower substrate 120 is provided with a plurality of gate and data lines 125 and 127 crossing each other to define the pixels. Also, a switching device for switching each pixel is provided at each crossing of the gate and data lines 125 and 127. For example, the switching device is formed of a thin film transistor 121 including a gate electrode, a semiconductor layer, and source and drain electrodes. Then, a gate pad 125a is provided at one side of each gate line 125, and a data pad 127a is provided at one side of each data line 127, wherein the gate and data pads 125a and 127a apply signals to the respective gate and data lines 125 and 127. Each pixel is provided with a pixel electrode 123, wherein the pixel electrode 123 of the lower substrate 120 is facing with the common electrode 103 of the upper substrate 110. The common electrode 103 and the pixel electrode 123 are formed of transparent conductive materials which are suitable for transmitting the light to the backlight 200.
Also, a photosensor 140 is formed in the black matrix region of the lower substrate 120, to sense the luminosity of external light and control the brightness of backlight. To expose the photosensor 140 to the external environment, corresponding portion in the black matrix of the upper substrate 110 is partially removed.
As shown in FIG. 2, according as the corresponding portion of the black matrix 105 is removed from the black matrix region of the upper substrate 110, the photosensor 140 of the lower substrate 120 is exposed to the external. At this time, the photosensor 140 is formed simultaneously when forming the thin film transistor 121.
FIG. 3 is a cross section view illustrating a thin film transistor and a photosensor included in a liquid crystal display device according to the related art.
As shown in FIG. 3, a substrate 120 includes a thin film transistor region (I) having a channel of p-type ion implantation region; a thin film transistor region (II) having a channel of n-type ion implantation region; and a photosensor region (III).
Referring to FIG. 3, a p-type semiconductor layer 163, an n-type semiconductor layer 164, and an n-type and p-type semiconductor layer 165 are formed at fixed intervals on the substrate 120 including a buffer layer 162. Then, a gate insulation film 166 is formed on the p-type semiconductor layer 163, n-type semiconductor layer 164 and n-type and p-type semiconductor layer 165. Also, a gate electrode 168 is formed on the gate insulation film 166 above the p-type semiconductor layer 163 and the n-type semiconductor layer 164.
Also, an insulating interlayer 170 including a contact hole to expose the semiconductor layer is formed on the gate electrode 168. Then, source and drain electrodes 172 are formed on the insulating interlayer 170, wherein the source and drain electrodes 172 are respectively connected with the p-type semiconductor layer 163, n-type semiconductor layer 164 and n-type and p-type semiconductor layer 165 through the contact hole to expose the semiconductor layer.
The n-type semiconductor layer 164 is formed such that its region being in contact to the source and drain electrodes 172 is provided with an n+-type ion implantation region 164a, its region being in contact to the gate insulation film 166 is provided with an ion non-implantation region 164b, and its region therebetween is provided with an n−-type LDD layer 164c. 
The p-type semiconductor layer 163 is formed without additional LDD layer and is formed such that its region being in contact to the source and drain electrodes 172 is provided with a p-type ion implantation region 163a and its region being in contact to the gate insulation film 166 is provided with an ion non-implantation region 163b. 
The n-type and p-type semiconductor layer 165 is formed such that its region being in contact to the source and drain electrodes 172 is provided with p+-type and n+-type ion implantation regions 165a and 165b, and its region being in contact to the gate insulation film 166 is provided with an ion implantation region 165c. 
On the ion-implantation process for forming the LDD layer, the n−-type LDD layer 164c of the n-type semiconductor layer 164 is formed by using the gate electrode formed on the gate insulation film as an ion implantation mask, instead of using a mask of photoresist pattern. However, the gate electrode as well as the mask of photoresist pattern is not formed in the photosensor region (III) on the ion implantation process for forming the LDD layer. Thus, n-type ions are doped in the ion implantation region 165c between the p+-type ion implantation region 165a and the n+-type ion implantation region 165b. 
In case of the photosensor region (III), if the ion implantation region 165c is formed between the p+-type ion implantation region 165a and the n+-type ion implantation region 165b, it is difficult to check the intensity of current in the photosensor region according to the intensity of external light.
In other words, if the external light becomes strong, it raises the intensity of current flowing through the source and drain electrodes, that is, p+-type and n+-type ion implantation regions 165a and 165b. Meanwhile, if the external light becomes weak, it lowers the intensity of current flowing through the source and drain electrodes. Accordingly, it is possible to check the intensity of current in the photosensor region according to the intensity of external light.
However, the related art photosensor region cannot check the intensity of current according to the intensity of external light since the n−-type ion implantation region 165c formed between the p+-type ion implantation region 165a and the n+-type ion implantation region 165b affects the intensity of current flowing through the p+-type and n+-type ion implantation regions, so that the sensing efficiency of photosensor region deteriorates. That is, as shown in FIG. 4, the related art photosensor region has the non-linear property of drain current according to drain-source voltage Vds, whereby it is difficult to check the difference of current according to the intensity of external light, precisely.